A resonant LC circuit which is composed of a conventional chip capacitor such as a multilayer ceramic capacitor, and an inductor, such as one composed of a coil of wire and a core of either compressed, sintered iron powder or a compressed, sintered, powdered ferrite, can be tuned to a desired center frequency either by abrading away a part of the capacitor, or adjusting the core of the inductor, or a combination of both. Abrading a capacitor reduces its capacitance and raises the resonant frequency of the circuit. However, it is difficult to stop the abrading operation at the exact instant required to adjust the resonant frequency of a circuit to a precise desired value. There is a tendency to overshoot the value being sought. If the abrading is conducted at a speed slow enough to achieve reasonable accuracy, the operation usually takes too long to be economical.
In a conventional circuit, the core of the inductor may be varied in position to achieve either an increase or decrease in the resonant frequency of an LC circuit. Thus, fine adjustment of resonant frequency is not solely dependent upon adjustment of the capacitor. However, adjusting the position of the core also may become uneconomical if too much time is consumed in doing it.
Microminiature hybrid circuits often include some form of printed capacitor and almost always include printed resistors. Inductors may also be printed if the inductance required is relatively small. In the past, printed inductors have usually comprised a flat spiral of vapor deposited metal with or without an associated layer of a magnetic material to increase the inductance. However, if the inductance required is large, the present state of the art is not conducive to use of a printed inductor. Instead, a mounted coil or other more conventional type inductor is used. If the hybrid type circuit is used as an LC resonant frequency circuit, most of the same problems appear in adjusting the circuit to a desired center frequency, as were present with the older type circuits.
One aspect of the present invention is an improved hybrid type microminiature LC circuit which includes a trimmable capacitor, a principal inductor which may be of conventional wire coil type or a printed type, and a second inductor, of the printed type, having a value of inductance which is small compared to that of the principal inductor. By "printed" is meant any flat-conductor type whether the conductive material is deposited by screen printing or by some other method such as deposition through a stencil. The second inductor may have its inductance changed in such small increments that a fine adjustment of inductance is possible. The second inductor is also of a type which can have its inductance increased by trimming. If the required inductance is not too large, the printed inductor may constitute the total inductance of the circuit.
The invention also includes a method of adjusting the above type circuit to tune it to a desired resonant center frequency. One aspect of the method comprises removal of part of the capacitor so as to lower the capacitance and, at the same time, raise the resonant center frequency of the circuit somewhat above the desired value in a rough or approximate manner, and then trimming the printed inductor so as to raise its inductance. The inductor is trimmed sufficiently in a fine adjustment to lower the resonant center frequency of the circuit to the desired value.
The invention also includes increasing the inductance of an inductor, in a fine adjustment, by a trimming operation and it also includes an inductor that is adapted to have its inductance raised by so trimming.